The present invention relates to a method and an apparatus for aligning a diffraction grating.
A diffraction grating is used for a spectroscopy module in an optical measuring device or an optical communication device. When assembling a spectroscopy module, an arranging direction of diffracted light needs to be aligned in an arranging direction of a photodetector such that several lights diffracted by a diffraction grating are received by the photodetector. In this case, since the arranging direction of the diffracted lights is generally perpendicular to the longitudinal direction of grooves of the diffraction grating, the photodetector and the diffraction grating need to be aligned.
However, the longitudinal direction of grooves of a diffraction grating is not easily detected. Therefore, to assemble a spectroscopy module that uses a diffraction grating, an active alignment is performed. That is, actual optical signals are sent to the diffraction grating, and the positional relationship between the diffraction grating and a photodetector is adjusted such that diffracted lights are received by the photodetector. This was very troublesome.
Therefore, it has been proposed to form a reference edge, which shows the longitudinal direction of grooves, on a substrate of a diffraction grating in advance. In this case, the reference edge is formed to be parallel or perpendicular to the longitudinal direction of the grooves of the diffraction grating. The arranging direction of a photodetector is then adjusted based on the direction of the reference edge.
A typical diffraction grating used in several devices is cut out from a large substrate, which is an original plate. A commercially available original plate of a diffraction grating is a replica manufactured by transferring a mold, which is formed by a ruling engine or by an interference exposure and a dry etching, on a resin layer, which is formed on a glass substrate. Thus, the parallelism, or perpendicularity, of the direction of grooves of the diffraction grating with respect to the direction of the sides of the substrate is not guaranteed.
Therefore, when cutting the diffraction grating, the direction of the grooves of the diffraction grating needs to be detected and adjusted to align with a predetermined reference direction by an aligning device.
In a conventional alignment device, the position of a diffraction grating is aligned while observing an image of grooves magnified by an optical system. However, if the pitch of the grooves of the diffraction grating is very small, such as if the pitch of the grooves is approximately 1 μm, the alignment of the diffraction grating is difficult.
In the case with alignment of a workpiece of a machining device, such as a dicing device, the workpiece is generally aligned by utilizing, for example, a reference edge or an alignment mark while observing two separate points of the workpiece. However, since several grooves having the same shape are arranged on a diffraction grating, it is difficult to align the diffraction grating accurately because one groove cannot be observed at two separate points.
Therefore, in a conventional diffraction grating alignment apparatus, a replica of a diffraction grating is manufactured as shown in FIG. 12. The replica has a region 72 and a region 73. The region 72 is formed by transferring a diffraction grating on part of a surface of a substrate 71. The region 73 is portion of the substrate 71 where the diffraction grating is not transferred. A boundary 74 between the region 72 and the region 73 is parallel to or perpendicular to the longitudinal direction of the diffraction grating. The diffraction grating is aligned based on the boundary 74. Also, a replica is manufactured as shown in FIG. 13. The replica of FIG. 13 has the region 72 and alignment marks 75a, 75b. The region 72 is formed by transferring a diffraction grating to the entire surface of a substrate. The alignment marks 75a, 75b are formed along the direction of the grooves of the diffraction grating. The diffraction grating is aligned based on the alignment marks 75a, 75b. 
However, in the conventional diffraction grating alignment method, a substrate that is larger than the size of the diffraction grating is used as described above. The diffraction grating is then transferred to part of the substrate and the boundary of the diffraction grating is used for alignment. Therefore, a substrate needs to be used that is larger than the size of the diffraction grating. Also, during a manufacturing process of a replica of a diffraction grating, it is difficult to transfer the boundary of the diffraction grating by the accuracy of higher than or equal to the pitch of the diffraction grating, which is several micrometers. Thus, the longitudinal direction of the grooves of the diffraction grating is not easily aligned at high degree of accuracy.
Further, a replica of a diffraction grating having an alignment mark is manufactured by forming alignment marks in advance in a mold of a diffraction grating and transferring the mold to a substrate. In this case, the mold of the diffraction grating is manufactured by a ruling engine or by a dry etching after performing an interference exposure. Therefore, it was sometimes particularly difficult to form alignment marks. That difficulty leads to increased cost.
Furthermore, transferring alignment marks that have a different size from the diffraction grating during the manufacturing process of a replica is likely to deteriorate the performance of the diffraction grating.